316 DISCOVERY CH. 



understand that what we see is only a limited view of the 

 universe. 



The astronomical evidence for the existence of invisible 

 bodies in the universe is not a matter of assertion, but 

 of interpretation of results based upon scientific inquiries. 

 Effects are measured and are referred to physical causes 

 which account for them in every detail, though the exact 

 mechanism of the action may not be understood. Galileo 

 discovered the laws of motion of falling bodies, and 

 Newton showed that the law of gravitation is sufficient 

 to prescribe the movements of the moon or a planet as 

 precisely as it does for the fall of a pebble to the earth, 

 yet the nature of gravitational attraction remains a 

 mystery still. 



From such a familiar case of relationship between fact 

 and inference as is afforded by a falling stone, it is easy 

 to pass to larger effects of gravitational action. For those 

 who have little faith in intangible evidence, the discovery 

 of the planet Neptune may be put in as a plea for con- 

 fidence in conclusions based upon it. The observation of 

 this planet in 1846 close to the position which mathe- 

 matics based on the law of gravitation had assigned to 

 it, afforded a striking instance of the ability of the law 

 to respond to any demands which could be made upon 

 it in the solar system. 



The planet Neptune happened to be an object bright 

 enough to be seen with telescopic aid, though it is quite 

 invisible to the naked eye. If no telescope had existed 

 when this new member of our system was discovered, the 

 actual test of the validity of the mathematical results 

 could not have been made the processes of the intricate 

 mathematical argument would have touched the planet 

 and followed its movements, but, in the absence of 

 ocular demonstration, the conclusion that a massive 



